The present invention relates generally to electric heating elements and, more specifically, to flexible heating cables which may be used in heating pads and electric bed covers.
In almost any type of electrically heated pad or bed covering, it is necessary to provide means to protect against dangerous overheat conditions. Many different types of thermostats and sensors are used for this purpose. One common approach which has been used for years in connection with heating wires or cables for use in underblankets involves an electric heating element which is provided with a coextensive heating or sensing wire separated from the other heating element by an insulating layer of meltable plastic, The plastic is selected so that it softens or melts and permits contact or at least a low resistance path between the two coextensive wires whenever there is an undesired rise in the temperature along the heating cable. The coextensive wires are typcially connected so that in the event there is a short circuit or high current flow between the wires, the resulting increased current flow will interrupt the power circuit to the heating element. This type of heating cable is described in my U.S. Pat. No. 3,628,093 and in the U.S. Pat. No. 3,591,765 to Owers.
One of the problems encountered in heat sensing circuits of the type disclosed in my eariler patent is the problem of providing switching means which will disable the circuit regardless of where along the length of the heating element wire the fault occurs. It can be readily understood that if the fault occurs at the end of the cable adjacent a common connection between the two coextensive wires, there will be little or no voltage drop to produce a current within the sensing circuit. Various approaches have been followed to overcome this problem. In general, such approaches have involved tapping the heating element at the center to provide two sections of heating wire each with a sensing wire to obtain suitable voltage drops in each of the sections of sensing wire or have involved the use of diodes which permit one to sense the short circuit during one-half of the power cycle while delivering power to the heating element during the other half of the power cycle.
The U.S. Pat. No. 4,251,718 to Cole discloses various embodiments in which rectifiers are used to assure adequate current flow through the sensing wire circuit regardless of where the fault occurs along the length of the heating wire. There are many British Specifications which disclose variations of this use of the diode to provide a sensing portion of each cycle in which the current through the sensing wire may be used to trigger the switching means for the circuit. Among these are British Patent Specification No. 1,155,188 and No. 1,585,921. Other British Specifications involving the concept of dividing the heating element intermediate its ends into two sections each of which has a separate sensing wire and cross-connecting the sensing wires to obtain the desired voltage drop are shown in U.K. Patent Specifications No. 1,588,783, 1,588,784 and 2,028,607.
It has also been known in the art to provide combinations of dual wound coils with coextensive but spaced single wound coils as disclosed in British Specification No. 1,456,684. In the '684 specification, the coils in the dual wound portion serve as a heater and are separated by a meltable insulating layer which, in the event of an overheat condition, the short circuit between the elements increases the current to blow a fuse and disable the circuit.
It has also been known in the art to employ dual element sensing wires of dual element heating wires which are wound coaxially on a single core. This type of construction is shown in the U.S. Pat. Nos. 2,846,560 to Jacoby et al and 3,493,727 Hosokawa et al.